Evidence for an in vivo role of insulin-like growth factor-binding protein-1 and -2 as inhibitors of collagen gene expression in vitamin C-deficient and fasted guinea pigs

Abstract

Acutely scorbutic and fasted (vitamin C-supplemented) guinea pigs exhibit decreased collagen gene expression associated with weight loss. We recently demonstrated that circulating insulin-like growth factor-binding protein-1 and -2 (IGFBP-1 and -2) are induced in these deficiencies, and that removal of IGFBP-1 and -2 from serum of such animals by specific antibodies reverses inhibition of cellular IGF-I-dependent functions, including collagen and DNA synthesis. Here we investigated the kinetics of induction of IGFBP-1 and -2 relative to suppression of collagen gene expression. Guinea pigs were fasted for 10-96 h, with 3-24% weight loss, or received an ascorbate-free diet for up to 4 weeks, with 5-28% weight loss during the third and fourth weeks (phase II of scurvy). In both deficiencies, there was noncoordinate regulation of collagen mRNA expression in tissues. Type I collagen mRNA concentrations in skin decreased rapidly after 5-10% weight loss and reached about 10% of normal levels, whereas in bone, there was a later, and not as extensive, decrease. The concentration of cartilage type II collagen mRNA decreased rapidly initially, but then remained at 40-50% of normal. Circulating IGF-I concentrations remained normal during the period when collagen gene expression was initially suppressed, although there was a later decrease. In contrast, mRNAs for IGFBP-1 and -2 and the circulating proteins were induced before or concomitantly with the suppression of collagen gene expression. The ability of fasted or scorbutic guinea pig sera to inhibit IGF-I action in cells increased in parallel with IGFBP activity ([125I]IGF-I binding), which, in turn, mainly reflected the concentration of IGFBP-1 in sera. Serum insulin may be the primary regulator of the IGFBPs. Its levels were decreased to 10-13% of normal when weight loss commenced, whereas cortisol levels, although increased, did not correlate with the induction of IGFBPs. The overall results taken together with our recent findings from cell culture experiments are compatible with circulating IGFBP-1 and -2 acting as inhibitors of collagen gene expression by blocking IGF-I action during fasting and phase II of vitamin C deficiency.